Well, what I would do is create a little PHP script that you run once you have a fairly busy board, which will count up the total words in ALL your messages, and divide this by the number of messages, to give you an average words/message figure. Now, you could then just use this figure to display the total amount of words on your board by doing something like:
$totalWords = $modSettings['totalPosts'] * 24.646;
Where 24.646 would be whatever the average amount was. You could run the little PHP script every so often to get a more accurate value (Indeed you could store this "value" in the SMF settings table)
The problem is the little PHP script would not be an easy write if you had lots of messages (>5000). You'd need to have it fetch maybe 500 at a time and add code to stop it timing out, as it would be a demanding job I suspect. Think this is best going in Mod Requests as it's not going to make it as a built in feature I'm afraid
